1. Field of the Invention
The present invention relates to spark plugs.
2. Description of the Related Art
Spark plugs used for ignition in internal-combustion engines including automobile engines generally comprise a metal shell to which a ground electrode is fixed, an insulator made of an alumina ceramic or the like, and a center electrode disposed inside the insulator. The insulator projects from the rear opening of the metal shell in the axial direction. A terminal metal fixture is disposed in the projecting part of the insulator and is connected to the center electrode via a conductive glass seal layer, resistor, etc. formed by a glass sealing step. A high voltage is applied to the terminal metal fixture to cause a spark over the gap between the ground electrode and the center electrode.
However, a combination of factors such as, e.g., an elevated spark plug temperature and an increased ambient humidity may cause the so-called flashover phenomenon in which high-voltage application results not in a spark over the gap but in a current flow on the surface of a projecting part of the insulator to cause a discharge between the terminal metal fixture and the metal shell. Primarily for the purpose of avoiding this flashover phenomenon, most of the spark plugs for general use have a glaze layer on the surface of the insulator. The glaze layer serves also to smoothen the insulator surface and thereby prevent contamination and to enhance the chemical durability or mechanical strength of the insulator.
In the case of the alumina insulator for use in spark plugs, a lead silicate glass glaze has conventionally been used which is obtained by incorporating a relatively large amount of PbO into a silicate glass to lower the softening point thereof. This glaze is described, e.g., in JP-A-8-279099. (The term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d.) However, the lead-containing glaze layer has a drawback that since the lead changes into Pb3O4 or Pb2O3 upon application of a high voltage, the insulation resistance of the glaze layer rapidly decreases and, hence, the glaze layer deteriorates in anti-flashover voltage. In addition, because of the increasing global concern about environmental conservation in recent years, glazes containing lead have gradually come to lose their acceptance. In the automobile industry, for example, where spark plugs are used in a large amount, investigations are being made with the aim of phasing out spark plugs with a lead-containing glaze layer in the future, in view of influences of waste spark plugs on the environment.
Leadless glazes such as borosilicate glasses and alkali borosilicate glasses have conventionally been studied as substitutes for the lead-containing glazes. However, these leadless glazes inevitably have drawbacks such as a high softening point and insufficient insulation resistance. Glazes for spark plugs have come to be required to have an insulating performance durable in severer environments, because the glaze layers on spark plugs are apt to heat up to higher temperatures than on ordinary insulating porcelains due to the conditions under which the spark plugs are used in engines, and because the voltage applied to the spark plugs is becoming higher with the recent trend toward performance advancement in engines. On the other hand, in automobile engines and the like, rubber caps are generally used for connecting the spark plugs to the engine electric equipment system. In this technique, tight contact between the insulator and the inner surface of the rubber cap is important for improving flashover-preventive properties.
An object of the invention is to provide spark plugs having a glaze layer which has a low lead content, can secure satisfactory tight contact between the insulator of the plug and the inner surface of a rubber cap, and is excellent in insulating property and flashover-preventive property.